There are many names for her. Arinna, Equestria, Astrum, Aether, and many more. For now though, I'll just stick with the artists originally given name, Galaxia, and she is a fan-version of Celestia and Luna's mother. According to legend (from what I guess, anyway) together with her husband, who also has many names (Comsos, Chronos, Morpheus, etc) they created the universe, and formed the magical world of Equestria. From there, they begot their two daughters, Celestia, who would have power over the sun, and Luna, would would bring about the night and give the moon it's faint pale light.

The original artist of this image is SpyrotheFox ([link]). I immediately fell in love with this artists depiction of Celestia and Luna's parents. And when I saw the vector, made by BlackM3sh ([link]), I just HAD to make a wallpaper featuring her! Hopefully I can make something better in the future though. My abstracts are starting to get old. =/